Backside illumination (BSI) image sensor chips are replacing front side illumination sensor chips for their higher efficiency in capturing photons. In the formation of the BSI image sensor chips, image sensors and logic circuits are formed on a silicon substrate of a wafer, followed by the formation of an interconnect structure on a front side of the silicon substrate. The interconnect structure includes a plurality of metal layers including bottom metal layer M1 through top metal layer Mtop. A passivation layer is then formed over the interconnect structure.
The formation of image sensor chips also involves a backside grinding, a p-type impurity implantation, and a steam anneal. The process is then continued to form backside contact pads, color filters, lenses, and the like, on the backside of the silicon substrate.
The steam anneal requires a high thermal budget, which adversely affects the pixel performance of the photo image sensors. When pixel sizes become increasingly smaller, the dark currents of the pixels become increasingly more important, and hence the adverse effect of the steam anneal becomes more severe.